1. Field of the Invention
The invention relates to the field of analog-to-digital converters and more particularly, to such converters fabricated with MOS technology.
2. Prior Art
The introduction of microcomputers, particularly those computers fabricated on a single substrate or chip, has made possible the use of these devices in many new applications. Since these computers are relatively inexpensive, they may be employed in home appliances, automobiles, etc.
One of the more difficult problems in employing these devices is interfacing them with sensors, transducers, and the like. Typically, sensors and transducers provide an analog voltage which must be converted to digital form for the microcomputer. Most often, this conversion is performed by bipolar, analog-to-digital converters (ADC) which are fabricated on separate chips. Ideally, these converters should be included on the same substrate with the microcomputer. The presently described ADC is suitable for fabrication on a silicon substrate which includes a microcomputer.
In less sophisticated computer applications, such as in home appliances and automobiles, it is desirable to employ only a single DC power supply. This DC power supply such as a 5 volt supply, may be employed both by sensors and transducers and also by the microcomputer and its peripheral circuits. For example, a potentiometer coupled to the 5 volt supply which may be manually adjusted, such as with a "dial" control may be employed. Thus, the ADC is required to convert, by way of example, a 5 volt analog signal into a digital signal when operating from a 5 volt supply. For MOS circuits this requires that the gates of the analog switches in the ADC be driven above the power supply potential, thereby necessitating bootstrapping circuits. Bootstrapping circuits require a considerable amount of substrate area, raising the cost of the ADC. The presently described ADC minimizes this problem by employing only a single bootstrapping circuit.
One of the major difficulties in fabricating an MOS ADC is the design of a practical voltage comparator. Ideally, the comparator should have high gain and high common mode rejection. Generally, these comparators are bipolar, direct-coupled differential amplifiers. It is difficult to realize a practical MOS differential amplifier, which operates from a single power supply potential. A unique chopper amplifier is employed in the presently invented ADC.